The present invention relates to a thermal overload relay that performs switching-over of contacts upon detection of an overcurrent, in particular to improvement in manipulation structure for returning to a trip state and an initial state.
Patent Document 1, Japanese Examined Patent Publication No. H7-001665, for example, discloses a thermal overload relay operated by detecting an overcurrent running in the main circuit.
The thermal overload relay of Patent Document 1 is described referring to FIGS. 14 and 15.
As shown in FIG. 14, the thermal overload relay comprises, in an insulator case 1 made of a resin mould, main bimetals 2 inserted in three phase electric circuit and wound with heaters 2a, a shifter 3 linked to free ends of the main bimetals 2 and movably supported on the insulator case 1, a switching mechanism 4 disposed in the insulator case 1 allowing linking to an end of the shifter 3, and a contact reversing mechanism 5 to changeover contacts by operation of the switching mechanism 4.
The switching mechanism 4 comprises, as shown in FIG. 15, a temperature compensation bimetal 7 to link to one end of the shifter 3, a release lever 8 to which the other end of the temperature compensation bimetal 7 is fixed, and an adjusting cam 12 connecting to the release lever 8 through a swinging pin 9 projecting at the lower end of the adjusting cam and abutting on the circumferential surface of an eccentric cam 11a of an adjusting dial 11 disposed rotatably in the insulator case 1 at the upper end of the adjusting cam 12. A rotation angle of the release lever 8 is set by varying an abutting position of the adjusting cam 12 on the circumferential surface of the eccentric cam 11a of the adjusting dial 11 through adjustment of the adjusting dial 11, thereby slightly rotating around a support shaft 13. Thus, the set current is adjusted by setting the rotation angle of the release lever 8.
The contact reversing mechanism 5 comprises a reversing spring 14 fixed at the lower end of the reversing spring to the release lever 8 and extending upwards, a slider 17 linking to the tip of the reversing spring 14 and moving a normally open side movable contact piece 15b and a normally closed side movable contact piece 16a, and a reset bar 18 to manually move the slider 17 to the normal position. The contact reversing mechanism 5 further comprises the above mentioned normally open side movable contact piece 15b and the normally closed side movable contact piece 16a, and a normally open side fixed contact piece 15a and a normally closed side fixed contact piece 16b, the both fixed contact pieces being disposed opposing the movable contact pieces. The reversing spring 14 is a member having a punched window 14a formed by punching a thin spring material and a curved surface with a disc spring shape around the punched window 14a. The reversing spring 14 is curved with a convex towards right hand side in a normal state shown in FIG. 14.
When the bimetal 2 bends with the heat generated by the heater 2a due to an overcurrent in the above-described structure, the shifter 3 shifts to the direction indicated by the arrow P in FIG. 14 caused by displacement of the free ends of the main bimetals 2. The shift of the shifter 3 pushes a free end of the temperature compensation bimetal 7 and rotates the release lever 8 counterclockwise around the swinging pin 9.
With progression of the counterclockwise rotation of the release lever 8, the reversing spring 14 deforms while bending with a convex towards the left hand side. The deformation of the reversing spring 14 moves the slider 17 linked to the tip of the reversing spring 14 so as to turn the normally open side movable contact piece 15b and the normally open side fixed contact piece 15a into a closed state and to turn the normally closed side movable contact piece 16a and the normally closed side fixed contact piece 16b into an open state. Based on the information of the closed state of the normally open side movable contact piece 15b and the normally open side fixed contact piece 15a, and the information of the open state of the normally closed side movable contact piece 16a and the normally closed side fixed contact piece 16b conducted by the reversing action of the switching mechanism 4, an electromagnetic contactor (not shown in the figures), for example, connected in the main circuit is opened to interrupt the overcurrent.
After the thermal overload relay is turned to a tripped state and an electric current in the electromagnetic contactor is interrupted, the main bimetal 2 cools down and returns to the initial state. If a reset operation is not conducted, the reversing spring 14 does not deform into the opposite direction with a convex towards the right hand side and the slider 17 does not move to the opposite direction holding the contact reversing mechanism 5 in the state unable to return to the initial state.
In order to return the contact reversing mechanism 5 to the initial state, the reset bar 18 is pushed-in to deform the reversing spring 14 in the opposite direction, thereby moving the slider 17 towards the opposite direction.
There are usually two reset states in the returning operation using the reset bar, i.e. a manual reset state and an automatic reset state, the two states being interchangeable. In the manual reset state, the reset bar is pushed in to return the contact reversing mechanism 5 to the initial state. In the automatic reset state, the reset bar is kept in the pushed-in condition and after the main bimetal 2 is cooled down, the contact reversing mechanism 5 automatically returns to the initial state.
If the reset bar 18 readily changes to the automatic reset state, and the electromagnetic contactor is not provided with self-hold circuit, the motor would restart when the main bimetal cools down after halting of the motor due to trip of the thermal overload relay.
In order to cope with this problem, a technology is known in which a projection linked to a head of the reset bar is provided around a case window for passing through the head of the reset bar. When the reset bar is interchanged from a manual reset state to an automatic reset state, the projection is broken and removed.
In the above-mentioned conventional technology, after breaking and removing the projection provided around the case window, the reset bar needs to be manipulated to change from a manual reset state to an automatic reset state. Thus, a complicated manipulation is required for the automatic resetting.
In addition, the reset bar readily interchanges between the manual reset state and the automatic reset state after breaking and removing the projection from the periphery of the case window, thus there is a possibility of wrong operation of the thermal overload relay.
In view of the above-described unsolved problems in the conventional examples, it is an object of the present invention to provide a thermal overload relay that allows interchange of a reset bar between a manual reset state and automatic reset state with a simple operation at multiple desired times, and avoiding wrong operation of the relay.
Further objects and advantages of the invention will be apparent from the following description of the invention.